Accurate characterization of semiconductor materials is of critical importance in the manufacture of semiconductor devices, since device performance typically is strongly dependent on such material parameters as carrier density, mobility, and lifetime, and on defect state density. Much effort has been expended in the past on developing semiconductor material characterization techniques, and a great variety of such techniques exist today. For a review, see, for instance, P. Blood and J. W. Orton, Reports on Progress in Physics, Vol. 41, pp. 157-257, (1978).
Among semiconductor material charactrization and measurement techniques, the noncontacting techniques are of special interest, since, inter alia, they permit acquisition of the pertinent information without causing any damage to the sample. As the size of individual semiconductor components, e.g., of a transistor in an integrated circuit, decreases and, on the other hand, the number of components on a single semiconductor chip increases, this capability becomes increasingly more important. Furthermore, noncontacting measurement techniques are typically simpler to automatize than contacting ones, and thus can be expected to play an increasingly prominent part in future automated VLSI (very large scale integration) production lines. For a review of noncontacting semiconductor test methods, see, for instance, G. L. Miller et al, Proceedings of the Topical Conference on Characterization Techniques for Semiconductor Materials and Devices, The Electrochemical Society, Inc., pp. 1-31, (1978), incorporated herein by reference.
Perhaps the most basic characteristic of semiconductor material for device application is its conductivity type, i.e., the polarity of the majority carriers. The conductivity of a sample can be either n-type, i.e., the majority carriers are electrons, or p-type, i.e., the majority carriers are holes. Since typically semiconductor devices depend on local alteration of conductivity type or level, knowledge of the actual conductivity type of a semiconductor wafer, and particularly of the near-surface volume of a semiconductor wafer in which devices are to be formed, prior to the commencement of processing steps such as doping is of great importance. This application is, inter alia, concerned with a method for conductivity type determination, and I will therefore next discuss such methods and related methods that yield, in addition to other information, also information on the conductivity type.
Several known effects can be used to determine the conductivity type of a semiconductor sample. These include the Hall, Seebeck, and Righi-Leduc effects, and the intraband Faraday effect. The forward direction of a p-n junction or Schottky barrier also indicates conductivity type. Of these effects, the Hall effect has probably been used most frequently for conductivity type determination. Hall effect measurements yield, in addition to information on the conductivity type, information on carrier mobility, and require the application of a magnetic field to the sample. For a general review of the Hall effect see, for instance, The Hall Effect and Related Phenomena, E. H. Putley, Butterworths, London, (1960).
Hall effect measurements for semiconductor characterization purposes are typically made by means of a four-point probe technique. A noncontacting technique is taught by U.S. Pat. No. 4,190,799, issued Feb. 26, 1980, to G. L. Miller and D. A. H. Robinson, for "Noncontacting Measurement of Hall Effect in a Wafer". The method disclosed in that patent comprises applying an RF voltage to a pair of concentric planar electrodes adjacent to the wafer, thereby capacitively coupling a radical RF current into the wafer. A magnetic field applied normal to the wafer produces a circular component of RF current, which, in turn, produces an axial RF magnetic field which is detected inductively by means of a pick-up coil adjacent to the wafer. This method is truly nondestructive and lends itself to automatization. On the other hand, it requires, for proper implementation, relatively sensitive (and therefore relatively complex) instrumentation.
The Seebeck effect, one of the thermoelectric effects, is the appearance of a voltage between two nonisothermal points of a sample. This effect is used extensively for conductivity type determination in the semiconductor industry, typically by means of a heated probe (or cooled probe) arrangement. See, for instance, John R. Yeager, Solid State Technology, Vol. 17(3), March 1974, pp. 14-16, and B. P. Kashnikov, Instruments and Experimental Techniques, Vol. 19(2), Part II, March-April 1976, page 575. This technique, in addition to the disadvantages inherent in contacting measurements generally, is subject to problems peculiar to the use of a contact probe at other than room temperature, such as probe oxidation (or frosting), as well as possible temperature-induced sample alteration.
Conductivity type determination by means of rectifying contacts is also well known in the art. See, for instance, H. Hakansson, The Review of Scientific Instruments, Vol. 43(9), pp. 1380-1381, (1972). In an exemplary application of this technique, contact is made to the semiconductor sample under test by means of a metal probe, and the conductivity type determined by observing the direction of the rectifying contact thus formed.
As is well known, the carrier concentration in a semiconductor sample can be altered by optical means. This effect, which forms the basis of photovoltaic measurements, can be used to measure minority carrier mobility and/or lifetime, as well as to determine the conductivity type of a semiconductor sample. Capacitive probe photovoltaic measurements of minority carrier diffusion length in semiconductor samples have, for instance, been reported by A. M. Goodman, Journal of Applied Physics, Vol. 32(12), pp. 2550-2552 (1961). Although the method requires no conductive contact with the sample, it is nevertheless not a "noncontacting" method. As described by Goodman, a thin transparent insulator with a transparent conductive layer deposited thereon is in physical contact with the illuminated sample surface, forming a parallel-plate probe capacitor with the sample. A reference capacitor is formed on the reverse sample side. The differential voltage between probe and reference capacitor is applied to a high input impedance preamplifier, the output of which is further amplified and then synchronously detected.
Use of the thin spacing-maintaining insulating layer between sample and electrode makes possible the formation of a relatively stable parallel plate probe capacitor of small plate spacing. Plate spacing stability is of importance since unwanted variations of spacing will add noise to the measurement, and small plate spacing is important since the signal amplitude is proportional to the inverse of the spacing. Such an experimental arrangement, in addition to requiring physical contact to the illuminated sample region, with the attendant previously mentioned disadvantages thereof, is also not well suited to studying the effects of changes of, e.g., variation of sample temperature or chemical ambient on the minority carriers. For instance, temperature changes typically cause a change of the dielectric properties of the capacitor dielectric, and the dielectric films tend to shield the underlying sample surface from exposure to the ambient atmosphere. However, such measurements are of considerable interest, since they can, inter alia, supply information on electronic surface states. We will refer to measurements of this type generically as "surface state spectroscopy".
A method similar to that described by Goodman (ibid) forms the basis of the standard test method for minority carrier diffusion length in Si, ASTM F391-78. See, for instance, 1981 Annual Book of ASTM Standards, Part 43, pp. 795-801, American Society for Testing and Materials, Philadelphia.
A method and apparatus for the contactless monitoring of carrier lifetime in semiconductor materials is disclosed in U.S. Pat. No. 4,286,215, issued Aug. 25, 1981, to G. L. Miller. The method disclosed in the patent comprises coupling the semiconductor sample into an LC resonant circuit which is the frequency-determining portion of a marginal oscillator adapted, typically, to maintain a constant amplitude RF voltage signal, and measuring the current required to maintain the signal constant. This current is related to the sample's conductivity, and, by illuminating the sample with light of appropriate wavelength, the conductivity of the sample can be modulated. The difference in the steady-state values of conductivity in the illuminated and the dark condition is proportional to the minority carrier lifetime.
As can be seen from the above discussion, currently available techniques for conductivity type determination in semiconductor samples typically either require contacting the sample surface under investigation, e.g., the heated probe technique, or yield more than the sought information at the expense of added apparative complexity, e.g., the noncontacting Hall effect technique. Currently available photovoltage measurement techniques typically also require physical contact with the sample surface under investigation, and typically are not well suited for carrying out surface state spectroscopy.
Because of the great importance of conductivity-type determination in semiconductor device manufacture a simple noncontacting, and thus completely nondestructive, technique therefore is of considerable commercial interest. In particular, such a technique that is easily adaptable to full automation and that can be used, if so desired, for routine monitoring of the conductivity type of the semiconductor wafers in a VLSI production line would fill a gap in available semiconductor material characterization techniques. And a simple noncontacting photovoltaic surface state spectroscopy technique applicable to a wide variety of experimental conditions would be of considerable scientific and technological interest.